A number of different oligomerisation technologies are known to produce α-olefins. Some of these processes, including the Shell Higher Olefins Process and Ziegler-type technologies, have been summarized in WO 04/056479 A1. The same document also discloses that the prior art (e.g. WO 03/053891 and WO 02/04119) teaches that chromium based catalysts containing heteroaromatic ligands with both phosphorus and nitrogen heteroatoms, selectively catalyse the trimerisation of ethylene to 1-hexene.
Processes wherein transition metals and heteroatomic ligands are combined to form catalysts for trimerisation, tetramerisation, oligomerisation and polymerisation of olefinic compounds have also been described in different patent applications such as WO 03/053890 A1; WO 03/053891; WO 03/054038; WO 04/056479 A1; WO 04/056477 A1; WO 04/056480 A1; WO 04/056478 A1; US 2005187418 A1; U.S. Complete patent application Ser. No. 11/130,106; WO 05/123884 A2 and WO 05/123633 A1.
The catalysts utilized in the abovementioned trimerisation, tetramerisation, oligomerisation or polymerisation processes all include one or more catalyst activators to activate the catalyst. Such activators are compounds that generate an active catalyst when combined with the catalyst.
Suitable activators include organoaluminum compounds, boron compounds, organic salts, such as methyl lithium and methyl magnesium bromide, inorganic acids and salts, such as tetrafluoroboric acid etherate, silver tetrafluoroborate, sodium hexafluoroantimonate, aluminate activators e.g. trityl perfluoro-tributyl aluminate, and the like.
Organoaluminum compounds which act as suitable activators include alkylaluminium compounds such as trialkylaluminum and aluminoxanes.
Aluminoxane activators are well known in the art and can be prepared by the controlled addition of water to an alkylaluminium compound, such as trimethylaluminium. In such process the alkylaluminium compounds are only partially hydrolysed to prevent or at least to reduce the formation of aluminium hydroxide during the preparation of aluminoxanes. Commercially available aluminoxanes consequently include unreacted alkylaluminium. The result is that commercially available aluminoxanes are usually mixtures of an aluminoxane and an alkylaluminium.
In this specification the term “aluminoxanes” is used to denote a compound represented by the general formulae (Ra—Al—O)n and Rb(Rc—Al—O)n—AlRd2 wherein Ra, Rb, Rc and Rd are independently a C1-C30 alkyl or halo-alkyl radical, for example methyl, ethyl, propyl, butyl, 2-methyl-propyl, pentyl, isopentyl, neopentyl, cyclopentyl, hexyl, isohexyl, cyclohexyl, heptyl, octyl, iso-octyl, 2-ethyl-hexyl, decyl, 2-phenyl-propyl, 2-(4-fluorophenyl)-propyl, 2,3-dimethyl-butyl, 2,4,4-trimethyl-pentyl and dodecyl; and n has the value of 2 to 50. Preferably n is at least 4.
The term “organylaluminium compound” is used herein to denote a compound with at least one organyl group bound to an aluminium atom.
The term olefinic compound as used herein denotes an olefin or any compound which includes a carbon to carbon double bond.
Methylaluminoxane (MAO) is a common aluminoxane catalyst activator used in the activation of especially Cr based oligomerisation catalysts. As MAO is produced by the reaction of trimethylaluminium (TMA) with water, commercially available MAO is in fact a mixture of MAO and TMA. Modified MAO (MMAO) is another such common activator and commercially available MMAO is also a mixture of MMAO and at least two different alkylaluminium compounds.
Depending on the process technology used by the various commercial producers of aluminoxanes, commercially available aluminoxanes include various concentrations of alkylaluminiums, and the applicant is not aware of commercial products in which the alkylaluminium content as a percentage of the total aluminium containing compounds exceeds 45 wt %. U.S. Pat. No. 7,141,633 mentions that commercially available alkylaluminoxanes may typically contain about 10 wt %, but optionally up to 50 wt % of the corresponding trialkylaluminium. In the case of a MAO and TMA mixture this would mean a TMA molar fraction (the moles of alkylaluminium per total molar amount of aluminium) of about 0.082 (8.2 wt %), but optionally up to 0.447 (44.65 wt %). On a TMA:MAO molar ratio basis (i.e. the moles of Al present in the TMA:the moles of Al present in the MAO), this would imply a molar ratio of about 0.0896:1, but optionally up to 0.8068:1.
Having said this, it is important to note that there exist some contradictory conclusions about the role and effect of residual TMA in MAO on metallocene catalysed polymerization of ethylene in the open literature. For example, Michiels et al. Macromol. Symp., 97, 1995, 171-183 investigated the effects of cocatalysts on ethylene polymerization activities for Cp2ZrCl2 systems resulting from mixing AlR3 (R=Me, Et, iBu) with MAO at different molar ratios. Their results show an increase in activity for increasing TMA:MAO ratios up to 0.3-0.5. Polymerization activities decreased at higher ratios. Resconi et al. Macromolecules, 1990, 23, 4489-4491 suggests that the cocatalyst in the metallocene-MAO system is actually TMA since MAO acts as a soluble carrier-activator of the ion pair formed. By using NMR spectroscopy Tritto et al. Macromolecules, 26(26), 1993 on the other hand demonstrated that MAO is a better alkylating agent than TMA and that MAO produces the active centers as cation-like species in titanocene catalysts. Contrary to Michiels et al. Macromol. Symp., 97, 1995, 171-183, Chien at al. J. Polym. Sci., Part A, Polym. Chem., 1991, 29, 459 showed that the activity and the molecular weight of the polymer decrease when the TMA content increases whereas Reddy Macromolecules, 1993, 26, 1180 again found enhanced activities upon TMA addition to MAO for ethylene polymerization using zirconocene catalyst systems. In all these papers MAO and TMA were premixed prior to contact with the transition metal catalyst.
Aluminoxane activators are costly to the effect that it impacts significantly on process economics of olefin oligomerisation technologies which utilize this class of activators. The inventors of the present invention have found a way of reducing the quantity of aluminoxane required to be used for the activation of oligomerisation catalysts by utilizing, in the specific manner of the present invention, a less costly compound, namely trialkylaluminium as additional activator component to the reaction.
The inventors of the present application have also demonstrated that the aforementioned desirable result cannot be achieved by simply adding more alkylaluminium to an aluminoxane activator (which generally already includes some alkylaluminium) and then adding this activator combination/mixture (as in the above open literature examples) to an oligomerisation catalyst. By following this procedure the activity of the catalyst has been shown to be reduced and it has further been shown that using an oligomerisation catalyst that has been activated by such a combination, leads to the formation of more solids (polyethylene (PE) and waxes) as compared to a process where no additional alkylaluminium is added to the activator. This is illustrated by comparative examples with two different aluminoxane activators, MAO-20Alk and MMAO-3A (see comparative example 3 below).
Most surprisingly, however the inventors of the present invention have found a method to use a reduced quantity of aluminoxane and the less costly alkylaluminium in a two stage activation of an oligomerisation catalyst which leads to higher catalyst activity and/or lower solids formation. Using this approach, the total Al:Cr requirement for effective catalysis is also reduced.